In liquid crystal display devices, organic electroluminescence (EL) display devices, and the like of the active matrix drive type, a thin-film transistor (TFT) is commonly used as a driving element for each sub pixel.
Recent years have seen active research and development of a configuration in which oxide semiconductor such as zinc oxide (ZnO), indium gallium oxide (InGaO), or indium gallium zinc oxide (InGaZnO) is used in the channel layer of a TFT element. TFT elements in which oxide semiconductor is used in the channel layer are characterized by having a small OFF current, having a high carrier mobility even in the amorphous state, and being able to be formed by a low-temperature process.
The properties of oxide semiconductor are likely to degrade due to damage from sputtering, etching, and the like. Therefore, in many cases, TFT elements in which oxide semiconductor is used employs a structure of the inverted-staggered (bottom gate/top contact) type and of the channel protection type as shown in FIG. 16C (e.g., see Patent Literatures 1, 2, 3, and 4).
A wiring pattern for the source electrode and the drain electrode of a TFT element having the above-described structure is formed in the following manner, for example. First, as shown in FIG. 16A, a photoresist 801 applied on a metal film 9018 is exposed to light via a photomask 701. Then, as shown in FIG. 16B, only the exposed portion of the photoresist 801 is removed, and thus photoresists 8011 and 8012 that have been patterned are formed. Furthermore, as shown in FIG. 16C, a portion of the metal film 9018 on which the photoresists 8011 and 8012 are not deposited is etched. Source electrode 9016 and drain electrode 9017 that have been patterned can be formed by thereafter removing the photoresists 8011 and 8012.